Microlithographic projection exposure apparatus

ABSTRACT

A microlithographic projection exposure apparatus contains an illumination system for generating projection light and a projection lens with which a reticle that is capable of being arranged in an object plane of the projection lens can be imaged onto a light-sensitive layer that is capable of being arranged in an image plane of the projection lens. The projection lens is designed for immersion mode, in which a final lens element of the projection lens on the image side is immersed in an immersion liquid. A terminating element that is transparent in respect of the projection is fastened between the final lens element on the image side and the light-sensitive layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of prior application Ser. No.10/917,371 filed Aug. 13, 2004, which is a continuation of InternationalApplication PCT/EP04/005816, with an international filing date of May28, 2004 and claiming priority of German Patent Application DE 103 24477. This application further claims priority of German PatentApplication DE 10 200 405 64 76 filed Nov. 23, 2004. The full disclosureof these prior applications is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to microlithographic projection exposureapparatuses such as are used for the production of microstructuredcomponents. The invention relates, in particular, to projection exposureapparatuses with an immersion projection lens.

2. Description of Related Art

Integrated electric circuits and other microstructured components areconventionally produced by several structured layers being applied ontoa suitable substrate, which may be, for example, a silicon wafer. Inorder to structure the layers, the latter are firstly covered with aphotoresist that is sensitive to light of a particular wavelength range,for example light in the deep ultraviolet spectral region (DUV).Subsequently the wafer that has been coated in this way is exposed in aprojection exposure apparatus. In this process, a pattern of diffractingstructures contained in a reticle is imaged onto the photoresist withthe aid of a projection lens. Since the lateral magnification in thiscase is generally less than 1, projection lenses of such a type arefrequently also referred to as reduction lenses.

After the photoresist has been developed, the wafer is subjected to anetching process, as a result of which the layer is structured inaccordance with the pattern on the reticle. The photoresist left behindis then removed from the remaining parts of the layer. This process isrepeated until all the layers on the wafer have been applied.

One of the most prominent objects in the development of the projectionexposure apparatuses is to be able to define lithographically structureshaving increasingly smaller dimensions on the wafer. Small structuresresult in high integration densities. This generally has a favorableeffect on the performance of the microstructured components producedwith the aid of apparatuses of such a type.

The size of the definable structures depends, above all, on theresolving power of the projection lens that is being used. Since theresolving power of the projection lenses is proportional to thewavelength of the projection light, one approach for the purpose ofdecreasing the resolving power consists in employing projection lighthaving shorter and shorter wavelengths. The shortest wavelengths thatare used at present are within the deep ultraviolet spectral region(DUV) and amount to 193 nm or occasionally even 157 nm.

Another approach for the purpose of decreasing the resolving powerstarts from the idea of introducing an immersion liquid having a highrefractive index into an immersion interspace that remains between afinal lens of the projection lens on the image side and the photoresistor another light-sensitive layer to be exposed. Projection lenses thatare designed for immersion mode, and that are therefore also designatedas immersion lenses, may attain numerical apertures (NA) of more than 1,for example 1.3 or 1.4. However, the immersion not only enables highnumerical apertures, and thereby an increased resolving power, but alsohas a favorable effect on the depth of focus. The greater the depth offocus, the less demanding are the requirements as regards an exactpositioning of the wafer in the image plane of the projection lens. Inthe broader sense, one also speaks of immersion when the light-sensitivelayer is covered by an immersion liquid without the final opticalelement of the projection lens on the image side necessarily beingimmersed in the immersion liquid.

The implementation of immersion mode, however, requires considerableadditional efforts in terms of structure and process engineering. Forinstance, it has to be ensured that the optical properties of theimmersion liquid are spatially homogeneous and temporally constant—atleast within the volume exposed to the projection light—even when thewafer with the photoresist applied thereon is moving relative to theprojection lens. At the present time the associated technologicalproblems have not been solved satisfactorily.

SUMMARY OF THE INVENTION

It is an object of the invention to specify a projection exposureapparatus designed for immersion mode that, with a simple structure,enables reliable and low-maintenance operation.

This object is achieved by means of a projection exposure apparatus withan illumination system for generating projection light. The apparatusfurther comprises a projection lens with which a reticle can be imagedonto a light-sensitive layer. The projection lens is designed forimmersion mode, in which a final lens of the projection lens on theimage side is immersed in an immersion liquid. According to theinvention, a terminating element is provided that is transparent inrespect of the projection light and is capable of being arranged betweenthe final lens on the image side and the light-sensitive layer in such away that it is immersed in the immersion liquid, at least with itsimage-side boundary surface.

The provision of a terminating element in the interspace between thefinal lens of the projection lens on the image side and thelight-sensitive layer has, inter alia, the advantage that constituentsissuing from the light-sensitive layer, or other contaminants arisingthere, are able to accumulate on the final lens on the image side atworst to a negligible extent, since the terminating element, inparticular the side thereof facing towards the light-sensitive layer,acts like a protective shield for the final lens on the image side. Inthis way the final image-side lens of the projection lens does not haveto be removed, but only the terminating element occasionally has to beremoved and mounted again after cleaning or exchange. Particularly ifthe terminating element is fastened to the projection lens from outsideand can be removed and mounted without disassembly of the projectionlens, the effort required for this remains comparatively low.

It is advantageous if an interspace that is capable of being filled upat least partially with immersion liquid remains between the final lensof the projection lens on the image side and the terminating element. Inthis further embodiment the terminating element is consequently immersedin immersion liquid on both sides, so that a slight refraction of lightoccurs also on the boundary surface of the terminating element on theobject side. The requirements as regards the adjustment and themanufacturing accuracy of the terminating element are correspondinglylow. For, especially in the case of large numerical apertures, even aterminating element with plane-parallel shape reacts very sensitively tomanufacturing defects, for example deviations from the nominalthickness, from the parallelism of the boundary surfaces and fittingerrors.

If the interspace is not filled completely with immersion liquid butonly partially, a gas-filled region remains between the immersion liquidand the final lens on the image side. This may be advantageous, forexample, when the projection lens is to be capable of being convertedbetween dry operation and immersion mode with as little effort aspossible. The fewer interspaces between fixed optical elements such aslenses or terminating elements are filled with immersion liquid, theless effect a change-over to immersion mode generally has on theadjustment of the projection lens. From this point of view it may evenbe advantageous not to allow the projection lens to be immersed in theimmersion liquid, but to leave a gas-filled region between theterminating element and the immersion liquid.

In an advantageous further embodiment of this configuration theprojection lens exhibits, for the purpose of introducing immersionliquid into the interspace between the final lens on the image side andthe terminating element, a first immersion device which is independentof a second immersion device for introducing immersion liquid into theinterspace between the final optical element and the light-sensitivelayer, so that no exchange of immersion liquid between the interspacesis possible. In this way it is ensured that contaminants issuing fromthe light-sensitive layer are unable to reach the final lens of theprojection lens on the image side via the immersion liquid.

In another advantageous configuration the terminating element has thesame refractive index as the final lens of the projection lens on theimage side and is optically coupled onto this lens with its object-sideboundary surface in such a way that projection light passing through theprojection lens is not refracted between the final lens on the imageside and the terminating element. The absence of any refraction betweenthe final lens and the terminating element results in a still smalleradjustment effort in the course of exchanging the terminating element.This can be realized, for example, by the final lens on the image side,the terminating element and also the immersion liquid located in betweenthem having the same refractive index.

A refraction of the projection light between the final lens on the imageside and the terminating element is also forestalled when theterminating element is optically contacted with the final lens on theimage side and both elements have the same refractive index. But opticalcontacting is also useful, in appropriate circumstances, in the case ofdiffering refractive indices, since in this way the final lens on theimage side is directly protected against contaminants by the terminatingelement which is optically contacted therewith. The terminating elementcan be optically contacted with the final lens on the image side inparticularly simple manner if the two boundary surfaces facing towardsone another are flat. An adjustment is then superfluous, since theposition of the terminating element along the optical axis and also theorientation in the plane perpendicular thereto are predetermined by theflat boundary surfaces.

In an advantageous configuration a first interspace that is capable ofbeing filled up with a first immersion liquid remains between the finallens of the projection lens on the image side and the terminatingelement. A second interspace that is capable of being filled with asecond immersion liquid remains between the terminating element and thelight-sensitive layer. The first interspace in this configuration isconsequently capable of being separated from the second interspace influidically sealing manner.

The first immersion liquid and the second immersion liquid do notnecessarily have to be different. But the use of differing immersionliquids has the advantage, inter alia, that the immersion liquids can beoptimally adapted to the specific conditions in the two interspaces. Thefirst immersion liquid, which is in contact with the final lens of theprojection lens on the image side, may for example have a very lowsurface tension, which for the second immersion liquid coming intocontact with the light-sensitive layer would no longer be acceptable.The first immersion liquid also does not definitely have to be so easilycleanable as the second immersion liquid, since it cannot becontaminated by the light-sensitive layer. An adaptation of the twoimmersion liquids from the point of view of chemical reactivity canfurthermore be undertaken. Since, for example, quartz glass andcalcium-fluoride crystals interact differently with adjoining liquids,the immersion liquids may be selected in such a way that they reactchemically as little as possible with the adjoining optical faces.

The terminating element in this configuration may be arranged indisplaceable manner. This makes it possible to distribute the shearforces acting on the immersion liquid, which arise as a result of therelative movement between the fixed projection lens and thelight-sensitive layer, between the first immersion liquid and the secondimmersion liquid in a practically arbitrary ratio.

If during a scanning operation of the projection exposure apparatus theterminating element is displaced synchronously with the light-sensitivelayer, constant distributions of force in the immersion liquids can beachieved over the entire scanning operation. This favors the formationof laminar flows of liquid, counteracting the formation of bubbles. Thisapplies, in particular, if the terminating element is displaced in aplane parallel to the light-sensitive layer.

With regard to minimal movements of the second immersion liquid, theterminating element and the light-sensitive layer may have likedisplacement speeds and displacement directions during a projection.There is then no longer any relative movement between the terminatingelement and the light-sensitive layer. The second immersion liquid thenremains with freedom of movement within the second interspace, despitethe common displacement movement of the light-sensitive layer and of theterminating element. For this reason it may also have a higher viscositythan the first immersion liquid.

If flows within the second immersion liquid still arise, these havetheir origin in inertial forces which appear in the course ofacceleration and deceleration during a scanning operation. If theseinertial forces are small enough, additional measures, which areotherwise necessary in order to keep the immersion liquid in theinterspace between the final lens on the image side and thelight-sensitive layer, can be dispensed with. Since these measuresgenerally promote bubbling, this configuration of the invention permitsbubbles in the region of the second immersion liquid to be very largelyavoided.

Although in the region of the first immersion liquid a relative movementoccurs between the projection lens and the terminating element in thecourse of a scanning operation, and as a result shear forces also arisethat act on the first immersion liquid, the terminating element may,unlike the wafer, be provided with an edge which prevents an undesirabledischarging of the first immersion liquid during scanning operation.Therefore also for the first immersion liquid no additional measures,such as, for instance, an incident flow of gases, are necessary in orderto prevent a discharging of the immersion liquid. Consequently bubblescannot arise, or cannot arise to an appreciable extent, in the firstimmersion liquid either.

The edge may be formed directly on the terminating element. The edge mayalso be part of a tank which is open in the direction towards the finallens of the projection lens on the image side and in the bottom of whichthe terminating element is arranged. The tank itself may then consist ofan opaque material, for example a metal or a crystal. Crystals such ascrystalline silicon, for example, have the advantage of being verydimensionally stable. On account of their low specific weight, theirstiffness and their low chemical reactivity, ceramics, for example basedon SiC, are also highly suitable as material for the tank. Theterminating element, which, for example, may consist of quartz glass,then forms merely a type of window at the bottom of the tank, throughwhich the projection light can pass.

In addition to having translational displaceability, the terminatingelement may also be capable of being tilted about a tilt axis parallelto the image plane. The tiltability constitutes an additional degree offreedom, with which movements of the first and second immersion liquidscan be influenced.

If the terminating element is, for example, tilted during a positioningmovement in an exposure intermission in such a way that the largestspacing between the terminating element and the light-sensitive layer issituated at the front in the direction of motion, a kind of wedge-shapedgap arises between the terminating element and the light-sensitivelayer. In this gap the second immersion liquid is entrained in thecourse of a positioning movement of the wafer over and beyond thelight-sensitive layer. The tilting movement in this case may be effectedin such a way that the shortest spacing between the terminating elementand the light-sensitive layer is so small that, as a consequence ofcohesive forces, the second immersion liquid is unable to pass throughthis gap. By virtue of this it is possible to displace the secondimmersion liquid over the surface of the wafer in very simple manner,also over greater distances and at greater speeds.

If an edge is provided on the terminating element, said edge should bedimensioned in such a way that in the course of a tilting movement ofthe terminating element the second immersion liquid is neverthelessprevented by the edge from discharging. Hence also in the course ofpositioning movements of the wafer, which are generally carried out atgreater speeds than the displacement movements during an exposure, thenecessity of entraining an immersion liquid with the aid of incidentflows of gases, or with similar measures, also ceases to apply.Consequently no appreciable formation of bubbles can occur, even in thecourse of positioning movements.

Alternatively, or also in addition to tiltability of the terminatingelement, there may be provision to displace the terminating elementperpendicular to the image plane. The spacing between thelight-sensitive layer and the terminating element may then, for example,be reduced so far that the immersion liquid remains in the secondinterspace solely by virtue of cohesive forces.

If the second immersion liquid cannot be kept in the vicinity of thefinal lens on the image side merely by virtue of the cohesive forces orby virtue of a tilting of the terminating element in the course of amovement of the wafer relative to the projection lens, a holding deviceknown as such may additionally be provided, which holds the secondimmersion liquid in the second interspace in non-contacting manner. Forthis purpose the holding device may comprise, for example, at least onegas nozzle, the discharge aperture of which can be directed towards thesecond immersion liquid.

In another advantageous configuration of the invention, at least thefirst interspace is arranged in a sealable container. The container maybe, for example, a type of housing which is penetrated by the projectionlens and which covers the entire supporting structure for the wafer. Asa result of evaporation of the immersion liquid, after some time asaturation vapor pressure arises within the container, which preventsmore immersion liquid from evaporating than condenses againsimultaneously. In this way, latent heat of evaporation may not arise atthose places where the immersion liquid comes into contact with asurrounding gas. Heat-sinks of such a type bring about an inhomogeneousdistribution of temperature and hence also an inhomogeneous distributionof refractive index of the immersion liquid, as a result of which theimaging properties are impaired.

In general, however, it takes a very long time until the saturationvapor pressure has arisen in the container merely as a result ofevaporation on the relatively small surface. Therefore the projectionexposure apparatus may comprise a supply device for supplying a vaporphase of the first immersion liquid in the container. Even if the firstimmersion liquid differs from the second immersion liquid, in general itis not necessary also to counteract an evaporation of the secondimmersion liquid, since as a consequence of the terminating element theinterface to a surrounding gas is very small.

A minimal cooling of the first immersion liquid is obtained when thevapor pressure of the vapor phase of the first immersion liquid in thecontainer is capable of being adjusted in such a way that it is at leastapproximately equal to the saturation vapor pressure of the vapor phaseof the first immersion liquid at the temperature prevailing in thecontainer.

Another possibility for forestalling fluctuations of temperature withinthe first immersion liquid as a consequence of local evaporationconsists in covering the first interspace in the upward direction atleast partially by means of a cover. The cover reduces the size of theinterface to a surrounding gas, on which cooling can occur as a resultof evaporation. This cover may, for example, be constructed in such away that only a small interspace filled with a gas remains between thecover and the first immersion liquid. The saturation vapor pressurearises relatively quickly there as a result of evaporation.

But the gas in the interspace may also be a special protective gas, thedensity of which is greater than the density of a surrounding gas. As aconsequence of the greater density, the protective gas is kept in theinterspace by the force of gravity. The protective gas shouldfurthermore be such that the solubility thereof in the second immersionliquid is as low as possible, preferably lower than 10⁻⁴ percent byvolume. In this way an undesirable diminution of the transmitting power,or changes in refractive index within the second immersion liquid thathave their origin in protective gas that has gone into solution, can becounteracted.

It is more favorable if the first immersion liquid directly adjoins thecover, so that an interface to a surrounding gas or to theaforementioned protective gas remains only where the cover isinterrupted.

This may be the case, for example, in the region of a recess which isprovided in the region of the final lens on the image side. The coverdoes not then touch the projection lens; at the same time, anequalisation of level may take place via the peripheral gap between thelens and the cover, so that the volume between the terminating elementand the cover is always completely filled with the first immersionliquid.

In this connection it is particularly favorable if the tank has an edgethat slides in sealing manner along the underside of the cover duringthe displacement movement of the terminating element. The edge prevents,on the one hand, a lateral discharging of the first immersion liquid. Atthe same time, with its upward-pointing lateral face it forms a sealacting in the direction towards the cover.

A liquid film consisting of the first immersion liquid and acting as alubricating seal may always remain between an upward-pointing lateralface of the edge and the cover. In order that this liquid film does notbreak away during a movement of the tank, a liquid reservoir may be sunkin the upward-pointing lateral face of the edge, out of which immersionliquid can subsequently flow. The immersion liquid may be under pressurein the liquid reservoir, so that the liquid film does not break awayeven when the relative movement between the tank and the cover generatesan underpressure in the marginal region of the edge. The pressure in theliquid reservoir may, for example, be generated by first immersionliquid being capable of being supplied under pressure to the liquidreservoir from outside the tank.

In order to collect small amounts of the first immersion liquid whichpass through the narrow gap between the edge of the tank and the coverand which display a lubricating action, on at least one outer side ofthe edge an overflow channel may be arranged which collects theoverflowing first immersion liquid and conducts it away.

In addition, in all the aforementioned configurations the terminatingelement may have has at least approximately the same refractive index asthe first and second immersion liquids. In this way it is ensured thatsmall maladjustments of the terminating element barely have any effecton the optical properties of said liquids. For this reason therefractive index of the terminating element may differ from therefractive index of the adjoining immersion liquids by no more than 1%and preferably by no more than 0.5%.

This can be obtained, for example, by the adjoining immersion liquidsbeing water and by the terminating element consisting of LiF.

It is particularly favorable if the terminating element is withoutrefractive power. “Without refractive power” here is to be understood tomean the property of an optical element of having no focusing ordefocusing effect. An example of such an optical element is aplane-parallel plate made of a homogeneous material. Such a plate doesin fact have an effect on the position of the image plane of theprojection lens and on the correction of the spherical aberration andmust to this extent be taken into account in the design of saidprojection lens. However, provided that a difference in refractive indexexists at the boundary surfaces, such a plate offsets beams impinging atan angle merely in parallel, the magnitude of the offset depending onthe angle of incidence. A terminating element without refractive poweris advantageous for the reason that, in this way, the requirementsregarding the adjustment thereof can be lowered further and consequentlythe adjustment effort after a cleaning or an exchange of the terminatingelement is again reduced.

Quartz glass, for example, enters into consideration by way of materialfor the terminating element. Since in the case of very short-waveprojection light in the deep ultraviolet spectral region, in particularat a wavelength of 157 nm, quartz glass and other conventional opticalmaterials are no longer sufficiently transparent, the use ofcalcium-fluoride, barium-fluoride or strontium-fluoride crystals or evenof mixed crystals such as calcium barium fluoride, for instance, hasbeen proposed as a substitute. These materials also enter intoconsideration for the terminating element. However, these cubic crystalsexhibit an intrinsic birefringence which results in an impairment of theimaging properties of the projection lens unless appropriatecountermeasures are taken.

For this reason the terminating element may comprise at least twomembers consisting of one of the stated crystals, the thicknesses ofwhich are so chosen, and the crystal lattices of which are so orientatedrelative to one another, that the influence of intrinsic birefringenceon projection light passing through is at least approximatelycompensated.

The members may, for example, be joined to one another seamlessly or mayeven be spaced from one another in the direction of the optical axis. Inthe last-mentioned case an interspace remaining between the members,which may be sealed all round, can likewise be filled with a liquid thatis transparent for the projection light. The faces adjoining theinterspace between the members do not definitely have to be flat but mayalso exhibit a curvature. If the face of the object-side partial elementpointing towards the interspace is concave and/or if the face of theimage-side members pointing towards the interspace is convex, then agood compensation of the intrinsic birefringence can be achieved alsofor beams that pass through the terminating element inclined at largeaperture angles.

In another advantageous configuration of the invention, at least oneface of the terminating element passed through by projection light isreworked by local removal of material with a view to correctingwavefront errors. This process, which is known as such, of compensatingfor wavefront deformations by slight removal of material, of the orderof magnitude of a few nanometres, can be employed particularlyeffectively in the case of the terminating element, since the latter islocated in the immediate vicinity of the image plane. In this connectionit is to be noted that the quotient of the refractive indices of theterminating element and of the immersion liquid is less than in drysystems without immersion liquid, so that correspondingly more materialhas to be removed in order to achieve the same effect as in dry systems.Particularly when the terminating element is a plane-parallel plate, thereworking of one or both faces turns out to be particularly easy.

In another advantageous configuration of the invention a protectivelayer that is impermeable in respect of immersion liquid is applied ontoat least one surface of the terminating element that is able to comeinto contact with immersion liquid. Such a protective layer isadvantageous in particular when fluoride crystals are used as materialfor the terminating element, since these crystals exhibit a relativelyhigh solubility in water. As a result of the application of a layer ofsuch a type, the material can be prevented from being corroded if wateror a substance containing water is used by way of immersion liquid. Theapplication of such a protective layer is advantageous not only inconjunction with a terminating element but quite generally in the caseof all optical elements consisting of cubic fluoride crystals,particularly in the case of calcium-fluoride crystals that may come intocontact with the immersion liquid.

In the course of the application of a protective layer, care should betaken to ensure that the latter completely covers the face to beprotected. Even extremely small openings in the protective layer mayresult in a penetration of immersion liquid and in the formation ofunderlayer corrosion. From this point of view, an ion-assisteddeposition of a protective layer of extremely high compactness(preferably greater than 98%) is advantageous, since as a result a localdetachment of the protective layer in the course of operation of theprojection exposure apparatus is largely prevented. The “compactness” ofa material here is to be understood to mean, for a given degree ofcrystallinity, the ratio of the specific density of the material to areference density at which the material is perfectly free from cavities.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and features of the invention will become apparentfrom the following description of the exemplary embodiments on the basisof the drawing. Shown therein are:

FIG. 1 a meridional section through a projection exposure apparatusaccording to a first exemplary embodiment of the invention in greatlysimplified, not-to-scale, schematic representation;

FIG. 2 an enlarged detail from the image-side end of a projection lenswhich is an integral part of the projection exposure apparatus shown inFIG. 1;

FIG. 3 a projection exposure apparatus according to a second exemplaryembodiment of the invention in a representation corresponding to FIG. 2,wherein the terminating element is optically contacted with the finallens on the image side;

FIG. 4 a projection exposure apparatus with an additional horizontalpartition according to a third exemplary embodiment of the invention ina detail representation corresponding to FIG. 1;

FIG. 5 a projection exposure apparatus with a displaced terminatingelement according to a fourth exemplary embodiment of the invention in adetail representation corresponding to FIG. 1;

FIG. 6 an enlarged detail from the image-side end of the projection lenswhich is an integral part of the projection exposure apparatus shown inFIG. 5, in a first displaced position of the support and of theterminating element;

FIG. 7 the image-side end of the projection lens from FIG. 6 in a seconddisplaced position of the support and of the terminating element;

FIG. 8 the image-side end of the projection lens from FIG. 6 with tiltedterminating element;

FIG. 9 an enlarged detail from the image-side end of a projection lensaccording to a fifth exemplary embodiment of the invention, wherein anadditional cover covers the first interspace;

FIG. 10 a further enlarged detail D from FIG. 9, in which the transitionbetween the cover and an edge of a tank receiving the first immersionliquid is shown;

FIG. 11 the image-side end of a projection lens according to a sixthexemplary embodiment of the invention, wherein the image-side face ofthe final optical element on the image side is curved;

FIG. 12 the image-side end of a projection lens according to a seventhexemplary embodiment of the invention, wherein a gas-filled interspaceremains between the final optical element on the image side and theimmersion liquid;

FIG. 13 the image-side end of a projection lens according to an eighthexemplary embodiment of the invention, wherein the terminating elementis divided up into two members along a curved face;

FIG. 14 the image-side end of a projection lens according to a ninthexemplary embodiment of the invention with terminating element dividedup in curved manner, which is completely received in immersion liquid;

FIG. 15 the image-side end of a projection lens according to a tenthexemplary embodiment of the invention, wherein a gas-filled interspaceremains between the terminating element and the immersion liquid;

FIG. 16 the image-side end of the projection lens shown in FIG. 15, butwith an immersion liquid filling the interspace between the finaloptical element on the image side and the terminating element;

FIG. 17 the image-side end of a projection lens according to an eleventhexemplary embodiment of the invention, in which, similar to theembodiment shown in FIG. 4, the terminating element is completelyimmersion in two different immersion liquids;

FIG. 18 the image-side end of a projection lens according to a twelfthexemplary embodiment of the invention, comprising with two completelyenclosed interspaces filled with immersion liquids;

FIG. 19 the image-side end of a projection lens according to athirteenth exemplary embodiment of the invention, wherein theterminating element is a thin plate on which an immersion liquid is heldwithout any confinement;

FIG. 20 the image-side end of a projection lens according to afourteenth exemplary embodiment of the invention, wherein theterminating element is formed by a deformable membrane which is fixed toa support for the light-sensitive layer to be exposed;

FIG. 21 the image-side end of a projection lens according to a fifteenthexemplary embodiment of the invention, wherein the terminating elementis formed by a deformable membrane which is fixed to a housing of theprojection objective;

FIG. 22 the image-side end of a projection lens according to a sixteenthexemplary embodiment of the invention, wherein the terminating elementis formed by a deformable membrane which is fixed in an adjustableframe.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows, in greatly simplified schematic representation, ameridional section through a microlithographic projection exposureapparatus denoted overall by 10 according to a first exemplaryembodiment of the invention. The projection exposure apparatus 10exhibits an illumination system 12 for generating projection light 13,which, inter alia, comprises a light source 14, illuminating opticsindicated by 16, and a diaphragm 18. In the exemplary embodiment that isrepresented, the projection light has a wavelength of 157 nm.

The projection exposure apparatus 10 further includes a projection lens20 which contains a plurality of lenses, only a few of which, for thesake of clarity, are represented in exemplary manner in FIG. 1 anddenoted by L1 to L5. By reason of the short wavelength of the projectionlight 13, the lenses L1 to L5 are fabricated from calcium-fluoridecrystals, which are still sufficiently transparent even at thesewavelengths. The projection lens 20 serves to image, in reduced manner,a reticle 24 which is arranged in an object plane 22 of the projectionlens 20 onto a light-sensitive layer 26 which is arranged in an imageplane 28 of the projection lens 20 and applied on a support 30.

The support 30 is fastened to the bottom of a tank-like container 32which is open in the upward direction and which is capable of beingdisplaced, in a manner not represented in any detail, parallel to theimage plane 28 with the aid of a displacing device. The container 32 isfilled up so far with an immersion liquid 34 that during operation ofthe projection exposure apparatus 10 the projection lens 20 is immersedin the immersion liquid 34 with its final lens L5 on the image side. Inthe exemplary embodiment that is represented, this lens L5 is alarge-aperture and comparatively thick lens; but here a plane-parallelplate is also to be encompassed by the term “lens”.

Via a supply line 36 and a drainage line 38 the container 32 isconnected to a conditioning unit 40 in which a circulating pump, afilter for cleaning immersion liquid 34, and also a temperature-controldevice are contained in a manner known as such and therefore notrepresented in any detail. The conditioning unit 40, the supply line 36,the drainage line 38 and also the container 32 form overall an immersiondevice denoted by 42 in which the immersion liquid 34 is circulated andin the process cleaned and kept at constant temperature. The immersiondevice 42 serves, in a manner known as such, for increasing theresolving power and/or the depth of focus of the projection lens 20.

In an interspace 43 remaining between the final lens L5 on the imageside and the light-sensitive layer 26 a terminating element 44 isarranged, the details of which will be elucidated in the following onthe basis of FIG. 2.

FIG. 2 shows the image-side end face 45 of the projection lens 20 in anenlarged detail from FIG. 1. In the enlarged representation it can bediscerned that the terminating element 44 has the shape of aplane-parallel plate with, for example, a circular or rectangular basearea and is separably and adjustably attached to the image-side end face45 of the projection lens 20 via two fastening elements indicated by 46and 48. With a view to illustrating the separability, a screw connection52 is indicated at the fastening element 46. For the purpose ofadjustment, fine-adjustment drives are provided which are indicated inFIG. 2 by micrometer screws 54, 55, 56 and 57.

The terminating element 44 comprises two plate-like members 44 a and 44b connected to one another, which have the same dimensions and whichbear against one another seamlessly. By reason of the short wavelengthof the projection light 13, the two members 44 a and 44 b are also eachfabricated from calcium-fluoride crystals. The crystal lattices of thetwo members 44 a, 44 b are orientated in such a way that a rotationallysymmetrical distribution of intrinsic birefringence results overall forthe terminating element 44. As an alternative to this, the terminatingelement 44 may also comprise more than two members with differingcrystal orientations. With a total of four plane-parallel members it is,for example, possible to compensate for the delay caused by intrinsicbirefringence to a very large extent for arbitrary directions ofincidence. Examples of the crystal orientations considered here can begathered, for example, from WO 02/093209 A2, from WO 02/099450 A2 andalso from US 2003/0011896 A1. The full disclosure of these applicationsis incorporated herein by reference.

In the case of the first exemplary embodiment shown in FIG. 2 theimmersion liquid 34 flows around the exchange element 44 from all sidesand is located, in particular, in the two gap-like interspaces 64 and 66which remain between the terminating element 44, on the one hand, andthe light-sensitive layer 26 or the final lens L5 on the image side, onthe other hand.

If emission of substances from the light-sensitive layer 26, or amechanical detachment of relatively small parts thereof, occurs duringthe operation of the projection exposure apparatus 10, then theterminating element 44 prevents contaminants contained in the immersionliquid 34 from being able to reach the flat image-side boundary surface68 of the final lens L5 of the projection lens 20 in unhindered manner.Although such a contact is also not totally ruled out, since the twogap-like interspaces 64 and 66 are not completely separated from oneanother, an exchange of liquid between the gap-like interspaces 64 and66 is at least made considerably more difficult by the terminatingelement 44 situated in between. For this reason, contaminated immersionliquid 34 practically does not ascend to the final lens L5 but ispredominantly supplied via the drainage line 38 to the conditioning unit40 and cleaned therein.

By reason of the protective effect of the terminating element 44 it ishardly still necessary to exchange the final lens L5 on account ofcontamination by contaminated immersion liquid 34 and, in connectiontherewith, to adjust it in elaborate manner.

An exchange of the terminating element 44, on the other hand, which isexposed in far higher measure to the contaminants emanating from thelight-sensitive layer 26, turns out to be comparatively easy. For, inorder to do this, the fastening elements 46 and 48 merely have to beloosened from the housing of the projection lens 20 with the aid of thescrew connections 52. The installation of the terminating element 44,which immediately follows a cleaning or an exchange, also requireslittle adjustment and is therefore easy. By reason of the design in theform of a plane-parallel plate, the terminating element 44 is withoutrefractive power and therefore has only comparatively little effect onthe imaging properties. This holds, in particular, also for the reasonthat the terminating element 44 floats in the immersion liquid 34, sothat, given suitable choice of the immersion liquid, only a very slightor even infinitesimal refractive effect arises at the boundary surfacesexposed to the projection light 13.

For all the optical elements that are fabricated from fluoride crystalsand that are able to come into contact with immersion liquid aprotective layer is applied, preferably at least on the optically activefaces, which protects the sensitive crystals from the immersion liquid.In the first exemplary embodiment represented in FIG. 2, protectivelayers 74, 76 and 78 of such a type are therefore applied onto the flatimage-side boundary surface 68 of the lens L5 and also onto the upperside 70 and onto the underside 72 of the terminating element 44.

The choice of the materials for the protective layers 74, 76 and 78depends, above all, on the immersion liquid that is employed, which inturn is chosen taking the wavelength of the projection light that isbeing used into account. In the case of light wavelengths of 193 nm,water, for example, enters into consideration by way of immersionliquid, which rapidly corrodes crystalline calcium fluoride on accountof the relatively high solubility thereof in water. In this case theprotective layers 74, 76, 78 may consist of SiO₂ or LaF₃, since thesematerials are not soluble in water.

In the case of a light wavelength of 157 nm, as used in the exemplaryembodiment described above, certain oils have a higher transparency thanwater and are therefore better suited by way of immersion liquid. Thelikewise highly transparent materials MgF₂ and LaF₃, for example, thenenter into consideration as materials for the protective layers 74, 76,78.

The protective layers 74, 76, 78 consisting of the stated materials canbe applied onto the boundary surfaces of the optical elements inquestion by evaporation coating in a vacuum.

FIG. 3 shows, in a representation based upon FIG. 2, a second exemplaryembodiment of a microlithographic projection exposure apparatus, whereinfor like parts use is made of the same reference numerals as in FIG. 2and for parts corresponding to one another use is made of referencenumerals augmented by 200. In the second exemplary embodiment theterminating element 244 is not separated from the final lens L205 on theimage side via an interspace 66 but is optically contacted with saidlens directly. Provided that the lens L205 and the terminating element244 are fabricated from a material having the same refractive index, theprojection light 13 passes through the boundary surface between theterminating element 244 and the final lens L205 without being refracted.The fastening by optical contacting has the advantage that no fasteningelements 46, 48 are required. In addition, after an exchange theterminating element 244 practically does not need to be adjusted, sincethe two flat boundary surfaces 270 and 268 facing towards one anotherpertaining to the terminating element 244 and to the lens L205,respectively, guarantee the correct arrangement by themselves in thecourse of optical contacting.

The image-side face 272 of the terminating element 244 is reworked atsome points 79 a, 79 b—represented in FIG. 3 on a greatly exaggeratedscale—by removal of material amounting to a few nanometres in such a waythat wavefront errors caused by the projection lens 220 are corrected.Since reworking methods of such a type are known as such, a moredetailed elucidation will be dispensed with.

FIG. 4 shows, in a representation based upon FIG. 1, a third exemplaryembodiment of a projection exposure apparatus, wherein for like partsuse is made of the same reference numerals as in FIG. 1 and for partscorresponding to one another use is made of reference numerals augmentedby 300. The projection exposure apparatus shown in FIG. 4 differs fromthat shown in FIG. 1 in that it comprises not just one but two immersiondevices 342 a and 342 b which are independent of one another. Thecontainer 332 here is subdivided horizontally by a partition 80 into twopartial containers 332 a and 332 b in such a way that the gap-likeinterspace 366 between the terminating element 44 and the final lens L5on the image side is arranged totally within the partial container 332a, and the gap-like interspace 364 between the terminating element 44and the light-sensitive layer 26 is arranged totally within the partialcontainer 332 b. The terminating element 44 is sunk with clearance in asuitably shaped cut-out 82 in the partition 80 between the partialcontainers 332 a and 332 b.

By virtue of the separation of the immersion liquids 334 a and 334 b inseparate immersion devices 342 a and 342 b, contaminated immersionliquid 334 b from the partial container 332 b is prevented from gettinginto the gap-like interspace 366 between the terminating element 44 andthe lens L5 and from being able to contaminate the latter in this way.

Further, the independent immersion devices 342 a, 342 b make it possibleto control the temperature of the immersion liquids 334 a and 334 bindependently. Thus the refractive of the liquids 334 a, 334 b may beindependently controlled with the help of the immersion devices 342 a,342 b. For example, the temperature of the immersion liquid 342 aadjacent the final lens L5 may be used for controlling the temperatureof the terminating element 44 and/or the final lens L5, and theimmersion liquid 334 b adjacent the light-sensitive surface 26 may beused for adapting the indices of refraction, for example to theimmersion liquid 334 a, to other immersion liquids or to thelight-sensitive surface 26.

In the following a fourth exemplary embodiment will be described on thebasis of FIGS. 5 and 6, which show schematically a detail from theimage-side end of a projection lens, and an enlarged detailedrepresentation thereof, respectively. Parts similar to those in FIGS. 1to 4 are denoted by like reference numerals, and parts corresponding toone another are denoted by reference numerals augmented by 400.

In the fourth exemplary embodiment a terminating element 444 is providedparallel to the image plane 28. The terminating element 444 is likewiseconstructed in the form of a plane-parallel plate which, however, isconsiderably larger than in the exemplary embodiments described above.In the exemplary embodiment that is represented, the terminating element444 has a rectangular basic shape and is sunk into a bottom 486 of atank 488. The tank 488, which, for example, may be fabricated frommetal, a ceramic or a crystal, serves for receiving a first immersionliquid 434 a, which in the exemplary embodiment that is represented isde-ionised water. An edge 490 of the tank 488 is so high that, given anappropriate filling height of the first immersion liquid 434 a, a firstinterspace 492 remaining between the final lens L5 on the image side andthe terminating element 444 is filled out completely with the firstimmersion liquid 434 a.

Between the terminating element 444 and the light-sensitive layer 26there remains a flatter second interspace 494 which is filled with asecond immersion liquid 434 b. In the exemplary embodiment that isrepresented, the second immersion liquid 434 b is likewise de-ionisedwater. The second interspace 494 is so flat that the second immersionliquid 434 b is hindered solely by cohesive forces from discharginglaterally out of the second interspace 494. The smaller the spacingbetween the terminating element 444 and the light-sensitive layer 26,the better do the cohesive forces hold the second immersion liquid 434 bin the second interspace 494.

In order to reduce the requirements as regards the parallelism of theterminating element 444 relative to the image plane 28, a materialhaving a refractive index that is as equal as possible to the refractiveindex of the surrounding immersion liquids 434 a, 434 b can be chosenfor the terminating element 444. In the case where use is made of waterby way of immersion liquid, LiF, which is still highly transparent atleast at a wavelength of 193 nm, is suitable, for example, by way ofmaterial for the terminating element 444. The difference in therefractive indices then amounts to only 0.0066.

If the projection light has a particularly short wavelength, for example157 nm, the first immersion liquid 434 a may also consist of afluorinated hydrocarbon that has a higher transmission than water atthese wavelengths. For the second immersion liquid 434 b the somewhatlower transmission is not too disadvantageous, inasmuch as the height ofthe second interspace 494 will generally be very low. In addition, waterhas the advantage that it corrodes the light-sensitive layer 26 lessseverely than is the case, for instance, with fluorinated hydrocarbons.

In the fourth exemplary embodiment the projection exposure apparatus isdesigned for scanning operation. This means that the reticle 24 isdisplaced in the object plane 22 during the projection. Synchronouslywith this, the support 30 with the light-sensitive layer 26 appliedthereon is also displaced parallel to the image plane 28. The lateralmagnification of the projection lens 420 determines the ratio of thedisplacement speeds and the displacement directions of the reticle 24and of the support 30.

For this purpose, the support 30 is clamped, with the aid of clampingelements 31 a, 31 b which are discernible in FIG. 5, on a displaceabletable 33 which is ordinarily designated as a wafer stage. The table 33can be displaced with great accuracy parallel to the image plane 28 in amanner known as such with the aid of actuating drives. The actuatingdrives are represented in simplified manner in FIG. 5 and are denoted by35 a and 35 b.

Manipulators 497 a, 497 b are fastened to the table 33, so that saidmanipulators jointly execute displacement movements of the table 33. Themanipulators 497 a, 497 b are connected to the tank 488 via actuatingarms 498 a, 498 b. The manipulators 497 a, 497 b are constructed in sucha way that they are able to move the tank 488 parallel to the imageplane 28 and relative to the table 33, to displace it perpendicularthereto, i.e. parallel to an axis OA, and also to tilt it relative tothe image plane 28. In the exemplary embodiment that is represented,tilting movements in particular are possible about two horizontal axeswhich extend perpendicular to directions of motion of the table 33 andperpendicular to the optical axis OA.

Furthermore, in FIG. 5 optional gas-discharge nozzles 499 a, 499 b arediscernible, with which a stream of gas can be directed onto aperipheral gap which is formed between the edge 490 of the tank 488 andthe light-sensitive layer 26.

The projection exposure apparatus show in FIGS. 5 and 6 operates asfollows:

During a scanning operation the table 33 is displaced together with themanipulators 497 a, 497 b in the direction of the arrow 496 b (see FIG.6) with the aid of the actuating drives 35 a, 35 b. The manipulators 497a, 497 b execute no actuating movements during this process, so that thetank 488 with the terminating element 444 sunk therein movessynchronously and at the same displacement speed and in the samedisplacement direction with the table 33 and consequently also with thelight-sensitive layer 26. In FIG. 6 this is indicated by an arrow 496 awhich has the same direction and length as the arrow 496 b. The tank 488consequently moves away beneath the projection lens 420 during thescanning operation together with the light-sensitive layer 26.

FIG. 7 shows, in a detail corresponding to FIG. 6, the relative positionof the tank 488 and of the light-sensitive layer 26, on the one hand,and of the projection lens 420, on the other hand, at the end of thescanning operation. Since the tank 488 moves synchronously, in paralleland at the same displacement speed as the light-sensitive layer 26during the scanning operation, no shear forces act on the secondimmersion liquid 434 b in the second interspace 494. The secondimmersion liquid 434 b therefore remains in the second interspace 494also during the displacement movements of the support 30. An incidentflow on the second immersion liquid 434 b with gases emerging from thedischarge nozzles 499 a, 499 b may therefore be reduced or may evenbecome superfluous. Hence one of the significant causes of the formationof bubbles in the second immersion liquid 434 b ceases to apply, eitherentirely or partially.

Since the first immersion liquid 434 a remains in the tank 488 solely byreason of the force of gravity, here too no incident flow with gases isrequired in order to prevent an escape of the immersion liquid duringthe scanning operation. Bubbles are also unable to arise to anappreciable extent as a result of the intermixing which the fixedprojection lens 420 brings about in the first immersion liquid 434 apassing by. Such an intermixing is entirely desirable, since in this waythe formation of relatively large temperature gradients is counteracted.

Overall it is possible for considerably diminished numbers of rejects tobe achieved in this way, since neither in the first immersion liquid 434a nor in the second immersion liquid 434 b can bubbles arise to anappreciable extent as a consequence of the displacement movements duringa scanning operation.

Between consecutive exposure cycles it is frequently necessary toreposition the support 30, with the light-sensitive layer 26 appliedthereon, with respect to the projection lens 420. The displacementspeeds in the course of these positioning movements are generallydistinctly higher than in the course of the movements during exposure.

If the tank 488 is exactly the same size as the light-sensitive layer 26applied on the support 30, during positioning movements of such a typethe tank 488 can be displaced just as synchronously and at the samespeed as has been described above in connection with the scanningoperations. In general, however, for various reasons it will beexpedient if the tank 488 has smaller dimensions parallel to the imageplane 28 than the light-sensitive layer 26 applied on the support 30.For example, the smaller the tank 488, the smaller also is the interfaceof the first immersion liquid 434 a to a surrounding gas. Accordingly,less heat of evaporation is also withdrawn from the first immersionliquid 434 a. This in turn has a favorable effect on a homogeneousdistribution of temperature, and hence of refractive index, within thefirst immersion liquid 434 a. From this point of view it would be idealif the tank 488 is only slightly larger than the region on thelight-sensitive layer 26 that is exposed overall during a scanningoperation.

For larger positioning movements, however, a smaller tank 488 means thatthe tank 488 cannot jointly execute this movement, at least not fully.In this case a relative movement between the light-sensitive layer 26and the terminating element 444 is unavoidable. This relative movementis generated by the manipulators 497 a, 497 b which are fastened to thetable 33.

In order to prevent formation of bubbles in the second immersion liquid434 b also during a faster positioning movement of the support 30, inthe fourth exemplary embodiment represented in FIGS. 5 to 7 the entiretank 488 can be additionally tilted with the aid of the manipulators.

In FIG. 8 the image-side end of the projection lens 420 is shown in arepresentation based upon FIGS. 6 and 7, the tank 488 having been tiltedby 2°. The tilt axis, which is denoted in FIG. 8 by TA, extendsperpendicular both to the optical axis OA and to the direction of motion496 b of the support 30. By virtue of the tilting movement of the tank488 about the axis TA, the second interspace 494, which has a constantheight during a scanning operation, is given the shape of a wedge-likeprism. In the tilted position of the tank 488 the end of the tank 488situated at the rear in the direction of motion 496 b is removed just sofar from the light-sensitive layer 26 that damage thereto is avoided.The cohesive forces now acting more strongly prevent, even at higherpositioning speeds, the second immersion liquid 434 b from emerging fromthe second interspace 494, whereas the support 30 moves away in thedirection of the arrow 496 b beneath the fixed or at worst slowly movingtank 488.

If during the scanning operations the spacing between thelight-sensitive layer 26 and the terminating element 444 is so smallthat a tilting movement about the tilt axis TA could involve damage tothe layer 26, alternatively a tilt axis may be chosen that extendsthrough the end of the tank 488 situated at the rear in the direction ofmotion 496 b. For the manipulators 497 a, 497 b this means that themanipulator 497 b raises the side of the tank 488 situated at the frontin the direction of motion by the requisite distance.

In order in the third exemplary embodiment shown in FIGS. 5 to 8 toprevent undesirable losses of heat of the first immersion liquid 434 aand of the second immersion liquid 434 b, the projection exposureapparatus exhibits a container 90 which tightly seals the spacesurrounding the immersion liquids 434 a, 434 b outwardly. Via an inlet92, water vapor which is generated by an evaporator 94 can be introducedinto the space surrounded by the container 90. The water vapor isintroduced until such time as the saturation vapor pressure applying atthe existing temperature is attained, at least approximately, within thespace surrounded by the container 90. In this way the immersion liquids,which here each consist of water, are prevented from graduallyevaporating, which would result in a cooling of the liquid at theinterfaces to the surrounding atmosphere. It will be understood that inthe event of a change-over to other immersion liquids other liquids alsohave to be evaporated correspondingly in the evaporator 94.

A fifth exemplary embodiment will be described in the following on thebasis of FIGS. 9 and 10, which show schematically a detail from theimage-side end of a projection lens and an enlarged detailedrepresentation D thereof, respectively. Parts similar to those in FIGS.1 to 4 are denoted by identical reference numerals; parts that havecounterparts in the fourth exemplary embodiment bear reference numeralsaugmented by 100.

In the fifth exemplary embodiment, in contrast with the third exemplaryembodiment shown in FIGS. 5 to 8, an additional cover plate 500 isprovided which almost totally covers the tank 588 in the upwarddirection. The covet plate 500, which does not have to be transparent,exhibits an opening 502, through which the image-side end of theprojection lens 520 is immersed in the first immersion liquid 534 a. Theedge 590 of the tank 588 slides along the underside of the cover plate500 in the course of a displacement movement of the tank 588 indicatedby an arrow 596 a.

The space between the cover plate 500 and the tank 588 is filled outcompletely with the first immersion liquid 534 a. For this purpose theopening 502 is so dimensioned that a peripheral gap 504 remains aroundthe image-side end of the projection lens 520, in which a liquid levelcan be adjusted.

FIG. 10 shows an enlarged detail D from the region of the edge 590. Inthe detail D it can be discerned that the edge 590 of the tank 588 isprovided on its upward-pointing lateral face with a peripheralwedge-shaped groove 506 and with a likewise peripheral rectangulargroove of larger cross-section, which constitutes a reservoir 508 forthe immersion liquid 534 a. The reservoir 508 and the wedge-shapedgroove 506, which is connected to the reservoir 508 via a duct 510, arealways filled with the first immersion liquid 534 a, so that a thinliquid film is formed on the upward-pointing lateral face of the edge590. This liquid film acts as a lubrication and in this way enableslow-friction and vibration-free sliding of the tank 588 along theunderside of the cover plate 500.

In order to ensure that the liquid film does not break away in thecourse of a movement of the tank 588 away beneath the cover plate 500,with gas bubbles thereby being introduced into the first immersionliquid 534 a, the first immersion liquid 534 a in the reservoir 508 andin the wedge-shaped groove 506 is under a slight overpressure. Thisoverpressure is generated by first immersion liquid 534 a beingconstantly supplied under pressure to the reservoir 508 via a supplyline 512. At the same time, excess first immersion liquid 534 a is ableto flow away via a drainage line 514. If the contact pressure generatedby the dead weight of the cover plate 500 does not suffice by way ofcounterpressure, the cover plate 500 can be additionally loaded, forexample with the aid of springs.

For the case where first immersion liquid 534 a emerges through asomewhat wider gap 516 on the outside of the edge 590, a peripheraloverflow channel 518 is provided which collects emergent first immersionliquid 534 a and conducts it away in a manner not represented in anydetail. A protective gas 519 that is heavier than air and that, forexample, may have the property of having only very low solubility inrespect of the first immersion liquid 534 a may be charged into theoverflow channel 518. As a result, molecules from a gas surrounding theentire arrangement, which impair the optical properties of the firstimmersion liquid 534 a in undesirable manner, are prevented from goinginto solution. The protective gas 519 in the overflow channel 518 is maybe renewed continuously, in order to counteract a gradual intermixingwith the surrounding gas.

The seal in the region of the edge 590 is of importance to the extentthat the support 30 of the light-sensitive layer 26 is not onlyfrequently displaced in a plane parallel to the image plane 28 but, witha view to diminishing imaging errors, can also be tilted slightly abouta horizontal axis. If the cover plate 500 is then not to be tiltedtogether with it, the seal in the direction towards the edge 590 must beso constructed that it guarantees sufficient imperviousness in relationto the cover plate 500 even in the case of relatively small tiltingmovements of the tank 588.

If the case may arise that the lubrication by the first immersion liquid534 a is not sufficient for a short time, for the cover plate 500 andthe edge 590 it is advisable to choose materials or coatings of theseparts that minimize or even entirely avoid a contamination of the firstimmersion liquid 534 a as a consequence of abrasion in the course ofstart-up. Aluminum oxide or diamond, for example, enter intoconsideration here as coatings.

The cover plate 500 has, on the one hand, the advantage that theoccurrence of waves in the tank 588 is prevented. On the other hand, thecover plate 500 limits the interface of the first immersion liquid 534 arelative to a surrounding atmosphere to the narrow peripheral gap 504which remains between the image-side end of the projection lens 520 andthe cover plate 500. In this way only very little heat is withdrawn fromthe first immersion liquid 534 a as a consequence of evaporation. Thisin turn diminishes the temperature gradient and hence therefractive-index gradient within the first immersion liquid 534 a, whichis formed in the course of heating by the projection light 13. In thecase of the second immersion liquid 534 b the problem of evaporationdoes not exist to an appreciable extent, inasmuch as the interfacebetween the second immersion liquid 534 b and a surrounding atmosphereis in any case very small.

In order to introduce the immersion liquids 534 a, 534 b into the firstand second interspaces 592 and 594, respectively, a relatively smallamount of the second immersion liquid 534 b may firstly applied onto thelight-sensitive layer 26. Subsequently the underside of the tank 588 ismounted on one side or parallel, and the second immersion liquid 534 bis expressed in bubble-free manner. The spacing between the tank 588 andthe light-sensitive layer 26 can later be adjusted precisely with theaid of the manipulators 497 a, 497 b.

Subsequently the cover plate 500 is laid over the tank 588. In order tofill the tank 588 with the first immersion liquid 534 a, the latter may,for example, be charged via the peripheral gap 504 which remains betweenthe projection lens 520 and the cover plate 500. However, it will beeasier if the edge 590 of the tank 588 is provided with an inlet andwith an outlet, via which the first immersion liquid 534 a can becharged into the tank 588 and removed therefrom. During operation of theprojection exposure apparatus the first immersion liquid 534 a may alsobe circulated continuously in a circuit, as elucidated in connectionwith the first three exemplary embodiments.

Of course, the arrangement shown in FIGS. 9 and 10 may likewise beaccommodated in a container 90, as shown in FIG. 5 with reference to thefourth exemplary embodiment. In this way the evaporation of theimmersion liquids is further reduced.

FIGS. 11 to 16 each show, in schematic representations based upon FIG.2, the image-side end of projection lenses according to furtherexemplary embodiments of the invention. In these Figures, for partssimilar to those in FIG. 2 use is made of the same reference numerals,and for parts corresponding to one another use is made of referencenumerals that are augmented by 600, 700, 800, 900, 1000 and 1100,respectively.

In the sixth exemplary embodiment shown in FIG. 11 the image-sideboundary surface 668 of the final lens L605 on the image side is notflat but curved in aspherically concave manner. For it has been shownthat, in the case of immersion objectives in particular, an asphericallycurved face in the immediate vicinity of the image plane 28 isparticularly well suited for the correction of higher-order imagingerrors. The prerequisite for this, however, is that the refractiveindices of the final lens L605 on the image side and of the immersionliquid 34 differ sufficiently from one another.

In the case of the projection lens 620 the terminating element 644 alsocomprises two members 644 a, 644 b which are fabricated fromcalcium-fluoride crystals or from similar cubically crystalline crystalswith suitably chosen crystal orientations. In the exemplary embodimentthat is represented, the final lens L605 on the image side consists ofquartz glass. As an alternative to this, the final lens L605 on theimage side may likewise consist of a cubically crystalline material. Thecrystal orientations of the crystals that the final lens L605 on theimage side and the members 644 a, 644 b consist of may then likewise beso aligned that a very extensive correction of the intrinsicbirefringence is achieved. The way in which a reciprocal birefringencecompensation can be achieved with three crystal orientations rotatedrelative to one another about the optical axis is described in detail inthe printed publications WO 02/093209 A2, WO 2/099450 A2 and US2003/0011896 A1 already mentioned above.

In the seventh exemplary embodiment represented in FIG. 12 the firstinterspace 792 remaining between the final lens L705 on the image sideand the terminating element 744 is not filled with immersion liquid 34completely but only partially. Therefore a gap-like interspace 793filled with a surrounding gas remains between the final lens L705 on theimage side and the immersion liquid 34.

This variant is particularly advantageous in the case of projectionlenses that are provided both for dry operation and for immersion mode.In order to have to perform as few modifications as possible to theprojection lens in the event of a change from dry operation to immersionmode, and vice versa, the optical conditions should change at as fewboundary surfaces as possible. In the case of the projection lens 720,for this reason the image-side face 768 of the final lens L705 on theimage side still adjoins a surrounding gas and not, for instance,immersion liquid 34.

On the other hand, also in the case of the projection lens 720 it isguaranteed that the terminating element 744 is surrounded by theimmersion liquid 34 on both sides. On account of the lowerrefractive-index quotient at the boundary surfaces of the terminatingelement 744, positional tolerances and manufacturing tolerances of theterminating element 744 may consequently only have a slight effect onthe imaging properties of the projection lens 720.

In the eighth exemplary embodiment shown in FIG. 13 the terminatingelement 844, which is plane-parallel overall, likewise comprises twomembers 844 a, 844 b which may consist of cubically crystallinematerials with differing crystal orientations. In contrast with theexemplary embodiments described above, the boundary surface between thetwo members 844 a, 844 b in the case of the projection lens 820 is notflat but curved. Furthermore, the two members 844 a, 844 b are notoptically contacted with one another directly but are spaced from oneanother, so that a narrow gap 899 remains between the members 844 a, 844b, which in the case of the projection lens 820 is filled with asurrounding gas.

In the case of the projection lens 820 only the image-side partialelement 844 b comes into contact with the immersion liquid 34. Thereforeit will generally be sufficient to exchange only the partial element 844b when required. The object-side partial element 844 a, on the otherhand, may be mounted on or in the projection lens 820 in such a mannerthat an exchange can only be carried out with major effort. Consequentlyonly the partial element 844 b constitutes an exchange element in thereal sense of the word.

The partitioning of the terminating element 844 into two members 844 a,844 b along a curved parting surface has the advantage that theimage-side partial element 844 b is likewise relatively insensitive tomanufacturing tolerances and positional tolerances. For, on the onehand, the image-side face is immersed in immersion liquid 34, so thatthe refractive-index quotient there is small. On the other hand, on theobject-side face of the partial element 844 b only relatively smallentrance angles arise by reason of the convex curvature thereof in thecase of light beams that pass through the terminating element 844 atlarge angles relative to the optical axis, so that manufacturingtolerances and positional tolerances are able to have less effect there.

The projection lens 920 shown in FIG. 14 according to a ninth exemplaryembodiment differs from the projection lens 820 merely by virtue of thefact that the immersion liquid 34 comes directly up against the finallens L905 on the image side. Therefore, unlike in the case of theprojection lens 820 shown in FIG. 13, both the gap 999 remaining betweenthe two members 944 a, 944 b and the first interspace 992 between thefinal lens L905 on the image side and the terminating element 944 arefilled up with immersion liquid 34. Positional tolerances andmanufacturing tolerances of the terminating element 944 have even lesseffect on the imaging properties of the projection lens in this variant.

Since, although the object-side partial element 944 a is exposed to theimmersion liquid 34, it is relatively well protected by the final lensL905 on the image side or by the image-side partial element 944 b, alsoin the case of the projection lens 920 an exchange of optical elementson account of contamination may be restricted to the image-side partialelement 944 b. However, the immersion liquid 34 reaching as far as thefinal lens L905 on the image side necessitates more extensivemodifications if a change-over from dry operation to immersion mode isdesired.

FIG. 15 shows an image-side end of a projection lens 1020 according to atenth exemplary embodiment of the invention. Unlike in the case of theexemplary embodiments described previously, the terminating element 1044is not immersed in the immersion liquid 34. The faces of the terminatingelement 1044 that are permeated by projection light consequently adjoina surrounding gas both on the object side and on the image side. Thisarrangement is also advantageous, in particular, in the case ofprojection lenses that are to be suitable both for dry operation and forimmersion mode. This is because the arrangement shown in FIG. 15requires particularly few modifications in the event of a change-overbetween the operating modes. On the other hand, the optical conditionsat the two boundary surfaces of the terminating element 1044 are largelyidentical. This is advantageous to the extent that, in particular,imaging errors that are generated by positional tolerances, for exampletilting movements, at the object-side boundary surface, are compensatedreally well by imaging errors acting in the opposite sense on theimage-side boundary surface.

The projection lens 1120 shown in FIG. 16 differs from the projectionlens 1020 shown in FIG. 15 by virtue of the fact that the firstinterspace 1192 between the final lens L1105 on the image side and theterminating element 1144 is filled out not with a surrounding gas butwith a liquid 1134. The image-side face of the terminating element 1114is therefore comparatively insensitive to manufacturing tolerances, inparticular fitting errors, of the terminating element 1144.

The second interspace 1194 between the terminating element 1144 and thelight-sensitive layer 26 can be filled up either partially withimmersion liquid 34, as is the case with the projection lens 1020 shownin FIG. 15. During dry operation, as shown by FIG. 16, thelight-sensitive layer 26 is not covered by immersion liquid.

FIG. 17 shows an image-side end of a projection lens 1220 according toan eleventh exemplary embodiment of the invention. A final lens 1209 isreceived in a lens mount 1210, which is connected to a housing 1202 ofthe projection lens 1220 such that it may not easily be replaced byanother lens. However, the lens mount 1210 may also be configured suchthat the final lens 1209 can be easily exchanged. The final lens 1209may be made of CaF₂, however, other lens materials such as SiO₂ arecontemplated as well.

The projection lens 1220 further comprises a terminating element 1211which may be made, for example, of amorphous quartz (SiO₂), CaF₂ oranother material having a high transmission at the wavelength used. Theterminating element 1211 is received in an element mount 1212 which isconnected to the housing 1202 of the projection lens 1220. Theterminating element 1211 is received in the element mount 1212 such thatit may easily be exchanged. Exchanging the terminating element 1211 maybe advantageous if it suffers from degradation caused by an immersionmedium 1213 during the exposure operation so that its optical propertiesdeteriorate. In order to facilitate an exchange of the terminatingelement 1211, it may be optically contacted to the element mount 1212,or it may be fixed to the element mount 1212 using a solder or a glue.

In the embodiment shown the terminating element 1211 is a plane-parallelplate. However, it is also envisaged to use a very thin plate or amembrane instead. Such a thin plate or a membrane has no significantoptical contribution, but effectively protects the final lens 1209.

In the interspace between the final lens 1209 and the terminatingelement 1211 there is a liquid 1214 having an index of refraction whichat least substantially equals the index of refraction of the immersionmedium 1213. Thus the effect of a possible deformation on one side ofthe terminating element 1211 is compensated for by a deformation on theother side. The liquid 1214 should be selected such that it does notcontaminate the final lens 1209 and the terminating element 1211. Inview of this, the liquid 1214 may be an oil, for example. If theimmersion medium 1213 is water, the liquid 1214 should have similaroptical properties. It is also possible to use the same liquid for theliquid 1214 and the immersion medium 1213.

The final lens 1209 and the terminating element 1211 are provided withprotective layers 1215 and 1216, respectively, for protection againstcorrosion caused by the liquid 1214. In a similar way the terminatingelement 1211 may be provided with a protective layer on its side facingtowards the immersion medium 1213 (not shown).

For achieving a certain degree of circulation of the liquid 1214, thehousing 1202 of the projection objective 1202 is provided with a supplyduct 1217 and an outlet 1218 for the liquid 1214. This makes it possibleto continuously rinse the interspace between the final lens 1209 and theterminating element 1211. Alternatively, the liquid 1214 may remaincontinuously in the housing 1202 of the immersion objective 1220. Inboth cases a temperature control of the liquid 1214 may be envisaged.Also for the immersion medium 1213 such a circulation and also atemperature control is envisaged. The temperature control of the liquid1214 and/or the immersion medium 1213 makes it possible to adjust therefractive index of the liquid 1214 and/or the immersion medium 1213.Since such a temperature control has been described above with referenceto the embodiment shown in FIG. 4, it will not be described here infurther detail.

The distance between the terminating element 1211 from the final lens1209 is determined such that the flow present in the immersion medium1213 and the liquid 1214 is, at least substantially, laminar. Such an atleast substantially laminar flow may be achieved by selecting thepressure of the liquid 1214 and/or the cross sections of the supply dug1217 and/or the outlet 1218, with a given distance between the finallens 1209 and the terminating element 1211, such that an at leastsubstantially laminar flow prevails within the liquid 1214.

FIG. 18 shows an image-side end of a projection lens 1320 according to atwelfth exemplary embodiment of the invention. The projection lens 1320differs from the projection lens 1220 shown in FIG. 17 in that theterminating element 1311 is connected to the housing 1302 of theprojection lens 1320, whereas the terminating element 1311 is connectedto a device 1319 for confining the immersion medium 1313.

FIG. 19 shows an image-side end of a projection lens 1420 according to athirteenth exemplary embodiment of the invention. This embodimentdiffers from the embodiment shown in FIG. 18 mainly in that theterminating element 1411 is formed by a thin plane-parallel plate whichis received in a ring-shaped element mount 1412. The terminating element1411 may be made, in spite of the high light intensities that occur inthe immediate vicinity of the light sensitive layer 26, of quartz glass.This is because the terminating element 1411 is moved together with thesupport 30, and therefore the high intensities are distributed in timeover a large area of the terminating element 1411. This ensures that aterminating element 1411 made of quartz glass will not be damaged byhigh light intensities.

The immersion liquid 1413 is connected via a channel 1418 to a reservoirsuch that the hydrostatical pressure in the interspace between the lightsensitive layer 26 and the terminating element 1411 remains constant.However, particularly if the terminating element 1411 is extremely thin,it may be advantageous to actively control the hydrostatical pressure,because this makes it possible to reduce a bending of the terminatingelement 1411 by adjusting the pressure in the liquid 1413. Forcorrecting aberrations, it may also envisaged to deliberately induce abending of the thin terminating element 1411.

Another difference to the embodiment shown in FIG. 18 is that the liquid1414 between the final lens 1409 and the terminating element 1411 is notconfined by walls or the like and does not circulate during exposureoperation, but remains at its place as a result of adhesion forces. Theliquid 1414 may only occasionally be exchanged.

FIG. 20 shows an image-side end of the projection lens 1520 according toa fourteenth exemplary embodiment of the invention. This embodimentdiffers from the embodiment shown in FIG. 19 mainly in that theterminating element is formed by a thin membrane 1511 instead of a thinplane-parallel plate. The membrane 1511 has elastic properties and maytherefore deform during movements of the support 30. If the immersionliquid 1513 and the liquid 1514 have the same index of refraction, andif the membrane 1511 has also the same index of refraction and/or isthin enough so that its refractive properties may be neglected,deformations such as shown in FIG. 20 have no substantial adverse effecton the imaging properties of the projection objective 1520. In the samemanner as a solid plate, the membrane 1511 ensures that the sensitiveimage-side surface of the final lens 1509 cannot be corroded by contactswith contaminated immersion liquid 1513.

FIG. 21 shows an image-side end of a projection objective 1620 accordingto a fifteenth embodiment. This embodiment mainly differs from theembodiment shown in FIG. 20 in that the membrane 1611 is not connectedto a ring-shaped element mount, which is moved synchronously with thesupport 30, but to the underside of the housing 1602 of the projectionobjective 1620. The membrane 1611 thus remains stationary duringscanning movements of the support 30. Also in this embodiment themembrane 1611 ensures that the sensitive image-side surface of the finallens 1609 cannot be corroded by contacts with contaminated immersionliquid 1613.

The membrane 1611 connected to the housing 1602 facilitates theintroduction of the liquid 1614 before the exposure operation begins.This particularly holds true if the image-side surface of the final lens1609 is concavely curved.

FIG. 22 shows an image-side end of a projection objective 1720 accordingto a sixteenth embodiment. This embodiment differs from the embodimentshown in FIG. 21 mainly in that the membrane 1711 is not immediatelyattached to the underside of the projection objective 1720. Instead, themembrane 1711 is received in a frame 1712, which may be circular orrectangular, for example. The frame 1712 is connected to the projectionobjective 1720 through fastening elements 1746, 1748 that may beconfigured similar to the fastening elements 46, 48 shown in FIG. 2.More particularly, the fastening elements 1746, 1748 may includefine-adjustment drives such that the frame 1712 can be, together withthe membrane 1711, accurately adjusted as desired. Thus it is possibleto adjust the membrane 1711 such that it is exactly parallel to thelight sensitive layer, or such that it forms a desired tilt angle withthe final lens 1709. Tilting the membrane 1711 may be advantageous withregard to the correction of certain aberrations.

Alternatively to what is shown in FIG. 22, the fastening elements 1746,1748 that enable the adjustment of the frame 1720 may be connected tothe support 30 such that the membrane 1711 moves synchronously with thesupport 30 during the exposure operation. Further, it is to beunderstood that only one or more than two fastening elements may beprovided in various configurations around the membrane 1711. Other meansfor adjusting the membrane 1711 are envisaged as well, for example byestablishing a desired pressure distribution in the liquids.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. The applicant seeks, therefore, tocover all such changes and modifications as fall within the spirit andscope of the invention, as defined by the appended claims, andequivalents thereof.

1. A projection lens of a microlithographic projection exposureapparatus, comprising: a final lens element and a terminating elementhaving no overall refractive power that is positioned between, butspaced apart from, the final lens element and an image plane of theprojection lens, and is adjustably mounted such that its position withrespect to the final lens is adjustable.
 2. The projection lensaccording to claim 1, wherein the terminating element is adjustablyattached to a housing of the projection objective.
 3. The projectionlens according to claim 2, wherein the terminating element is adjustablyattached to an outer surface of the housing.
 4. The projection lensaccording to claim 1, comprising an adjustable fastening member forattaching the terminating element.
 5. The projection lens according toclaim 4, wherein the fastening member comprises a micrometer screw. 6.The projection lens according to claim 1, wherein the terminatingelement comprises a surface having rotationally non-symmetrical surfacedeformations for correcting wavefront errors.
 7. The projection lensaccording to claim 1, wherein the terminating element is aplane-parallel plate.
 8. A projection lens for a microlithographicprojection exposure apparatus, comprising: a final lens element and aterminating element having no overall refractive power that ispositioned between, but spaced apart from, the final lens element and animage plane of the projection lens, and comprises a surface havingrotationally non-symmetrical surface deformations for correctingwavefront errors.
 9. A microlithographic projection exposure apparatus,comprising an illumination system for generating projection light andthe projection lens of claim 1, wherein the terminating element isimmersed in an immersion medium.
 10. The apparatus according to claim 9,wherein an interspace formed between the final lens and the terminatingelement is at least partly filled with a liquid.
 11. The apparatusaccording to claim 10, wherein the terminating element is aplane-parallel plate.
 12. The apparatus according to claim 10, whereinthe terminating element is a deformable membrane.
 13. The apparatusaccording to claim 12, wherein the membrane is received in an adjustableframe.
 14. The apparatus according to claim 12, wherein the membrane isattached to a housing of the projection lens.
 15. The apparatusaccording to claim 11, wherein the membrane is attached to a support fora light-sensitive layer.
 16. The apparatus according to according toclaim 10, comprising a temperature controller for independentlycontrolling the temperature of the immersion medium and the temperatureof the liquid.
 17. The apparatus according to according to claim 16,comprising first temperature changing means for changing the temperatureof the immersion medium and second temperature changing means forchanging the temperature of the liquid.
 18. The apparatus according toclaim 10, wherein the immersion medium is liquid.
 19. The apparatusaccording to claim 18, comprising a first loop for circulating theimmersion medium and a second loop for circulating the liquid.
 20. Theapparatus according to claim 19, wherein the first temperature changingmeans are arranged in the first loop and the second temperature changingmeans are arranged in the second loop.
 21. The apparatus according toclaim 10, wherein the terminating element and the final lens are spacedapart by a first distance, and the terminating element and alight-sensitive surface are spaced apart by a second distance, theimmersion medium is liquid, the apparatus comprises a first loop forcirculating the immersion medium and a second loop for circulating theliquid, each loop comprising a supply channel and an outlet channelhaving cross-sections, wherein at least one parameter in the groupconsisting of: cross-sections of said supply channels and outletchannels, pressure of the immersion medium and the liquid, and first andsecond distances are determined such an at least substantially laminarflow prevails in the liquid and the immersion medium.